1. Field of the Invention
The invention pertains to a spring plate for a strut unit.
2. Description of the Related Art
DE 42 03 658 C2 describes a spring plate especially for a MacPherson strut unit. This spring plate has a profile which accommodates the final turn of a helical spring and thus centers the spring radially and prevents it from rotating. The spring plate has an out-of-round circumferential area, against which the helical spring is radially supported. As can be seen in FIG. 4 of DE 42 03 658 C2, almost the entire inside diameter of the final turn of the helical spring rests against the spring plate.
This 3-dimensional design of a spring plate is highly favorable in theory, because the force of the helical spring is supported over a large area. In addition, because of the out-of-round profile of the contact surface for the spring, an anti-rotation function can also be provided. The problem, however, is that the dimensions of the final turn of a spring cannot be kept within the necessary tolerances without increasing manufacturing costs to an unacceptable level, which means that the combination of a helical spring with the spring plate described in the state of the art is very difficult to realize.
When the helical spring does not rest optimally on the spring plate, strong stresses develop in the spring plate. Under certain conditions, these elevated stresses might be absorbed by the use of materials with appropriate load-bearing capacities. Another major problem is the noise which is produced when the helical spring changes its length as a result of its elastic movement. The noise problem can be solved in many cases by the use of a shim of plastic between the helical spring and the spring plate. This solution, however, does not eliminate the cause of the noise, only the effects. In older vehicles, the shim can also wear out, so that the noise then becomes clearly audible again.